Fix hist() handling of fractional $step

This fixes sf.net bug #3588182 by making the $xvals
returns in list context alway be double and updates
the documentation to reflect this.  Also fixes a
logic error in the calculation of the value for $step
if not provided.
Fix sf.net bug #3585811 "conv1d bad value broken"

Now a warning is generated if conv1d() is used on data
with the badflag set.  An explicit caution is also added
to the documentation.